Owing to recent weight reduction and miniaturization of electrical appliances, a lithium secondary battery having a higher energy density is desired more than ever before. Furthermore, increased applications of the lithium secondary battery also require improvement in the battery performance.
At present, the positive electrode of the lithium secondary battery utilizes a metal oxide salt, such as lithium cobalt oxide, lithium nickel oxide, or lithium manganese oxide. The negative electrode of the lithium secondary battery utilizes a carbonaceous material, such as coke, artificial graphite, or natural graphite, alone or in combination.
In such a lithium secondary battery, it is known that a solvent in an electrolyte solution may decompose on the surface of the negative electrode, and thereby the storage characteristics or the cycle performance of the battery are deteriorated.
Ethylene carbonate, however, less decomposes on the surface of the negative electrode. In addition, a decomposition product of ethylene carbonate forms a relatively good protective film on the surface of the negative electrode. Thus, ethylene carbonate has been conventionally and widely used as the main solvent in an electrolyte solution of a nonaqueous electrolyte secondary battery. However, even when ethylene carbonate is used, the electrolyte solution slightly and continuously decomposes during charge and discharge. Thus, this may decrease the coulombic efficiency of the battery.
To solve these problems, it is known that a small amount of an agent for forming a protective film, for example, vinylene carbonate, is added to the electrolyte solution (for example, Japanese Unexamined Patent Application Publication No. 6-52887). The agent for forming the protective film decomposes to produce a decomposition product on a surface of a carbonaceous negative electrode during initial charge and discharge. The decomposition product thus produced forms a good protective film and thereby improves the storage characteristics or the cycle performance of the battery. For this reason, the agent for forming the protective film is often used in the lithium secondary battery.
On the other hand, in recent years, a next-generation nonaqueous electrolyte secondary battery has been proposed and gained attention. This battery includes a metal, such as tin or silicon, or oxide thereof which absorbs and discharges lithium ions, as a new negative electrode material which has much higher charge and discharge capacity per unit mass or unit volume than the carbonaceous negative electrode (Solid State Ionics. 113-115.57(1998)).
Particularly, the nonaqueous electrolyte secondary battery having an electrode formed by depositing a thin film of the active material that absorbs or discharges lithium, such as a silicon thin film or a tin thin film, on a collector by a CVD method, sputtering, evaporation, thermal spraying, or plating exhibits high charge and discharge capacity and excellent charge and discharge cycle performance. In such an electrode, the thin film of the active material is divided into columns by cracks formed in the thickness direction. The bottom of each column adheres to the collector. A gap around the column relaxes stress generated by the expansion and contraction of the thin film during charging/discharging cycles. This relaxation reduces the stress, which may cause the detachment of the thin film of the active material from the collector. Thus, the battery exhibits excellent charge and discharge cycle performance (Japanese Unexamined Patent Application Publication No. 2002-279972).
However, the negative electrode material made of a metal, such as silicon or tin, or of an alloy or an oxide containing the metal element is, in general, more reactive with various electrolytes, organic solvents, and additives in the electrolyte solution than the conventional carbonaceous negative electrode. Thus, an electrolyte additive has been desired from which a protective film adaptable to these new negative electrode materials is formed.